1. Field of the Invention
The present invention relates to an integrating light source, and more particularly, to a light source for use in a film scanner employing an image sensor.
2. Background of the Invention
Apparatus such as document and film scanners, which have image sensing arrays to produce an electronic image signal by sensing an original, frequently employ diffuse light sources to illuminate the original. U.S. Pat. No. 4,864,408, issued to Bridges, Sept. 5, 1989, shows a charge-coupled device image sensor in a film-to-video player for sensing light transmitted through a frame of film from a light box. The light box houses a tungsten halogen bulb at one end and a diffuser at the opposite end adjacent the film. An intermediate opaque pyramidal surface, or "flag", prevents direct light from the bulb reaching the diffuser. The pyramidal flag helps to provide a uniform light intensity across the surface of the diffuser.
For a film scanner, operating at normal film projection rates, e.g. 24 frames per second, an intense uniform source of light is required to illuminate the original one line at a time. For optimum scratch suppression, it is also desirable for the light to be diffuse and nearly uniform in angular distribution (i.e., Lambertian). The light source described in U.S. Pat. No. 4,864,408 is designed for a different application, i.e., imaging a stationary frame, and consequently suffers from the drawback that the incandescent light source is not as bright as often desired for a high resolution film scanner. Intense light sources such as Xenon arc lamps and lasers are not produced in a linear configuration like an elongated incandescent lamp and are inconvenient for locating inside a light box.
A film scanner with an intense illumination source is described in U.S. Pat. No. 4,868,383, issued to Kurtz et al, Sept. 19, 1989. A linear light source for a film scanner disclosed therein includes a source of an intense beam of light, such as a Xenon arc lamp, and an elongated cylindrical integrating cavity having diffusely reflective walls. The cavity includes an input port through which the intense beam is introduced into the cavity and an output slit parallel to the long axis of the cylindrical integrating cavity for emitting a uniform line of light.
To provide optimum illumination in a film scanner, it is important to tailor the distribution and color balance of the light from the slit. Many imaging applications require a diffuse source of visible light to produce uniform illumination of an image plane. Thus most lamps need to be enclosed in or directed towards a diffusing means. The diffuser may be a frosted transparent or translucent plate in front of the source or a hollow enclosure or integrating volume such as described in U.S. Pat. No. 4,868,383. In that disclosure, the integrating cavity is preferably machined from a block of diffusely reflecting polytetrafluoro ethylene plastic. Alternatively, the enclosure may incorporate, or be coated with, diffusely reflecting compounds to further diffuse the light. The compounds generally have a relatively uniform reflectance with respect to wavelength in the visible spectrum. For instance, the cavity may be constructed from a material such as aluminum, with a diffusely reflective coating on the internal surfaces, such as barium sulfate based paint.
In U.S. Pat. No. 4,864,408, the housing and baffle parts forming the box are anodized blue. The light reflected off the inner surfaces of the box picks up a blue cast. Because the box is blue, all other colors are absorbed. This is beneficial for the silicon based CCD utilized in this application because devices of this type have relatively poor responses in the blue region. The concentration of dye used in the anodizing process determines how much blue light can be reflected and how much of other colors are absorbed. In this way the lamp box's spectral output can be tuned to the requirements of the CCD without the use of additional spectral filtering parts.
In many cases, the chosen illumination source will produce insufficient radiation in portions of the usable electromagnetic spectrum, but produce substantial radiation elsewhere. The unwanted radiation can be attenuated by absorptive or reflective filtering, and the usable radiation passed on to the imaging system. Further filtering is used to achieve the desired illumination spectrum. This method of spectral shaping, however, results in a waste of potentially useful radiated energy.